This invention relates to linear actuators and, more particularly, to an improved linear actuator of the type disclosed in copending U.S. application Ser. No. 579,432 filed May 21, 1975 by Messrs. Halfhill and Brunner as a divisional of copending U.S. application Ser. No. 486,408 filed on July 8, 1974 now U.S. Pat. No. 3,922,718.
The unique linear actuator disclosed in copending applications Ser. Nos. 486,408 and 579,432 makes use of the principle that a roller frictionally engaged with the cylindrical surface of a drive shaft will be rotated about its axis by rotation of the drive shaft when such axis is parallel to the axis of the drive shaft, and will additionally be moved linearly in a direction parallel to the axis of the drive shaft when the roller axis is oblique to the axis of the drive shaft.
In general terms, the linear actuator disclosed in the aforesaid copending applications Ser. Nos. 486,408 and 579,432 includes a drive shaft having a cylindrical surface, means for rotatably mounting the drive shaft to a support frame for rotation of the drive shaft about its axis, means for rotating the drive shaft about its axis, a carriage to be driven, a roller having a peripheral surface, means for mounting the roller to the carriage with the roller being rotatable about a first axis and pivotable about a second axis perpendicular to the first axis, means for mounting the carriage to the support frame with the carriage being movable relative to the support frame along the predefined linear path and with the roller being in frictional engagement with the cylindrical surface of the drive shaft whereby the roller is caused to rotate about its first axis by rotation of the drive shaft when the first axis is parallel to the axis of the drive shaft and is additionally caused to move along the predefined linear path during rotation of the drive shaft when the first axis is oblique to the axis of the drive shaft, and means for controllably pivoting the roller about its second axis to control movement of the roller and thus the carriage along the predefined linear path.
As disclosed in the aforesaid copending applications Ser. Nos. 486,408 and 579,432, the linear actuator may be included in and form part of a magnetic disk drive. More specifically, disk drives generally include a drive spindle for rotating one or more magnetic recording disks. A head carriage is associated with each disk and may include two electromagnetic heads, one for each surface of the disk. Since information is recorded on the disk in concentric tracks which are spaced very closely adjacent one another, it is necessary to provide a linear actuator for the head carriage that is capable of moving the carriage and thus heads thereon to and from selected tracks on the disk at high speed and with great precision. Energization of the linear actuator to cause movement of the head-carriage assembly in the appropriate direction and speed is controlled by a suitable servo control system.
It is apparent that the precision and speed required in positioning the head-carriage assembly of a disk drive leaves little room for error. Positioning errors may occur if either the peripheral surface of the roller or the cylindrical surface of the drive shaft wears non-uniformly. Non-uniformity of wear in the roller might result in vibrations which could adversly effect position control. Vibrations may result in special positioning problems when the servo control system is a closed loop system, such as the type having a track following capability.
More specifically, data is recorded on concentric tracks on the disk surface as the disk is rotated about its axis. Due to the fact that the disk is supported and driven by mechanical components, it is apparent that the tracks of data will not be precisely concentric, but will each contain a slight degree of eccentricity or "run-out." If a head were positioned over a track and remained absolutely fixed as the disk rotated in order to recover data on the track, it is clear that the absolute concentric following of an otherwise slightly eccentric track might cause some errors in data recovery, or at least periodic reduction in the amplitude of data read from the disk.
In order to overcome this problem, some servo systems have been designed with a "track following" capability in order for the heads to be able to follow a track precisely notwithstanding the slight eccentricity thereof. The frequency of "run-out" or degree of eccentricity must be within the bandwidth capabilities of the servo system in order for the servo to properly control the linear actuator in order for the heads to peroperly follow each track. Thus, any non-uniform wear of the roller that would result in vibrations within the servo bandwidth might cause track following errors.
As alluded to above, non-uniform wear of the cylindrical surface of the drive shaft might also result in positioning errors. More particularly, such non-uniform wear may result in grooves being formed in the cylindrical surface. In addition to possibly causing unwanted vibrations, the grooves would cause a detenting action in carriage movement. This detenting action might result in the heads being positioned over the wrong track altogether. In this respect, positioning errors due to detenting are potentially more serious than those resulting from most vibrations experienced in head carriage positioning.
It would be desirable, therefore, to provide a linear actuator of the above-described type wherein the wear experienced by the roller peripheral surface and the cylindrical surface of the drive shaft will be primarily, and preferably substantially entirely, borne by the roller peripheral surface, and wherein the wear on such peripheral surface will be substantially uniform.